Compressive Line Sensing (CLS) imaging system is a compressive sensing (CS) based imaging system with the goal of developing a compact and resource efficient imaging system for the degraded visual environment. In the CLS system, each line segment is sensed independently; however, the correlation among the adjacent lines (sources) is exploited via the joint sparsity in the distributed compressing sensing model during signal reconstruction. Several different CLS prototypes have been developed. This paper discusses the development of a pulsed CLS system. Initial experimental results using this system in a turbid water environment are presented.

The compressive line sensing imaging system adopts distributed compressive sensing (CS) to acquire data and reconstruct images. Dynamic CS uses Bayesian inference to capture the correlated nature of the adjacent lines. An image reconstruction technique that incorporates dynamic CS in the distributed CS framework was developed to improve the quality of reconstructed images. The effectiveness of the technique was validated using experimental data acquired in an underwater imaging test facility. Results that demonstrate contrast and resolution improvements will be presented. The improved efficiency is desirable for unmanned aerial vehicles conducting long-duration missions.

The Compressive Line Sensing (CLS) active imaging system has been demonstrated to be effective in scattering mediums, such as turbid coastal water through simulations and test tank experiments. Since turbulence is encountered in many atmospheric and underwater surveillance applications, a new CLS imaging prototype was developed to investigate the effectiveness of the CLS concept in a turbulence environment. Compared with earlier optical bench top prototype, the new system is significantly more robust and compact. A series of experiments were conducted at the Naval Research Lab's optical turbulence test facility with the imaging path subjected to various turbulence intensities. In addition to validating the system design, we obtained some unexpected exciting results – in the strong turbulence environment, the time-averaged measurements using the new CLS imaging prototype improved both SNR and resolution of the reconstructed images. We will discuss the implications of the new findings, the challenges of acquiring data through strong turbulence environment, and future enhancements.

The Compressive Line Sensing (CLS) active imaging system has been demonstrated to be effective in scattering mediums, such as coastal turbid water, fog and mist, through simulations and test tank experiments. The CLS prototype hardware consists of a CW laser, a DMD, a photomultiplier tube, and a data acquisition instrument. CLS employs whiskbroom imaging formation that is compatible with traditional survey platforms. The sensing model adopts the distributed compressive sensing theoretical framework that exploits both intra-signal sparsity and highly correlated nature of adjacent areas in a natural scene. During sensing operation, the laser illuminates the spatial light modulator DMD to generate a series of 1D binary sensing pattern from a codebook to “encode” current target line segment. A single element detector PMT acquires target reflections as encoder output. The target can then be recovered using the encoder output and a predicted on-target codebook that reflects the environmental interference of original codebook entries. In this work, we investigated the effectiveness of the CLS imaging system in a turbulence environment. Turbulence poses challenges in many atmospheric and underwater surveillance applications. A series of experiments were conducted in the Naval Research Lab’s optical turbulence test facility with the imaging path subjected to various turbulence intensities. The total-variation minimization sparsifying basis was used in imaging reconstruction. The preliminary experimental results showed that the current imaging system was able to recover target information under various turbulence strengths. The challenges of acquiring data through strong turbulence environment and future enhancements of the system will be discussed.

A fine structure underwater imaging LiDAR (FSUIL) has recently been developed and initial field trials have been conducted. The instrument, which rapidly scans an array of closely spaced, narrow, collimated laser pulses into the water column produces two-dimensional arrays of backscatter profiles, with fine spatial and temporal resolution. In this paper a novel method to derive attenuation profiles is introduced. This approach is particularly attractive in applications where primary on-board processing is required, and other applications where conventional model-based approaches are not feasible due to a limited computational capacity or lack of a priori knowledge of model input parameters. The paper also includes design details regarding the new FSUIL instrument are given, with field results taken in clear to moderately turbid water being presented to illustrate the various effects and considerations in the analysis of the system data. LiDAR waveforms and LiDAR derived attenuation coefficients are analyzed and compared to calibrated beam attenuation, particulate scattering and absorption coefficients. The system was field tested during the NATO Ligurian Sea LIDAR & Optical Measurements Experiment (LLOMEx) cruise in March 2013, during the spring bloom conditions. Throughout a wide range of environmental conditions, the FSUIL was deployed on an in situ profiler obtaining thousands of three-dimensional LiDAR scans from the near surface down to the lower thermocline. Deployed concurrent to the FSUIL was a range of commercially available off-the-shelf instruments providing side-by-side in-situ attenuation measurement.

In recent years, a compressive sensing based underwater imaging system has been under investigation: the Compressive Line Sensing (CLS) imaging system. In the CLS system, each line segment is sensed independently; with regard to signal reconstruction, the correlation among the adjacent lines is exploited via the joint sparsity in the distributed compressive sensing model. Interestingly, the dynamic compressive sensing signal model is also capable of exploiting the correlated nature of the adjacent lines through a Bayesian framework. This paper proposes a new CLS reconstruction technique through the integration of these different models, and includes an evaluation of the proposed technique using the experiment dataset obtained from an underwater imaging test setup.

The compressive line sensing (CLS) active imaging system was proposed and validated through a series of test-tank experiments. As an energy-efficient alternative to the traditional line-scan serial image, the CLS system will be highly beneficial for long-duration surveillance missions using unmanned, power-constrained platforms such as unmanned aerial or underwater vehicles. In this paper, the application of an active spatial light modulator (SLM), the individually addressable laser diode array, in a CLS imaging system is investigated. In the CLS context, active SLM technology can be advantageous over passive SLMs such as the digital micro-mirror device. Initial experimental results are discussed.

The compressive line sensing (CLS) imaging system adopts the paradigm of independently sensing each line and jointly reconstructing a group of lines. This system achieves “resource compression” and is compatible with the conventional push-broom line-by-line sensing mode. This paper discusses the development of a prototype system to enable the experimental study the CLS imaging system. The results from an initial turbidity cycle experiments are presented.

Originally proposed in SPIE DSS’13, the compressive line sensing (CLS) imaging system adopts the paradigm of
independently sensing each line and jointly reconstructing a group of lines. Such system achieves “resource
compression” and is still compatible with the conventional push-broom operation mode. This paper attempts to extend
the CLS concept, originally developed to effectively acquire scene intensity images in a scattering medium, to 3D scene
reconstruction through the adoption of a temporal-spatial measurement matrix. The sensing model is discussed.
Simulation results are presented as part of this work.

High bandwidth (10 to 100 Mbps), real-time data networking in the subsea environment using free-space lasers has a potentially high impact as an enabling technology for a variety of future subsea operations in the areas of distributed sensing, real-time wireless data transfer, control of unmanned undersea vehicles, and other submerged assets. However, the development and testing of laser networking equipment in the undersea environment are expensive and time consuming, and there is a clear need for a network simulation framework that will allow researchers to evaluate the performance of alternate optical and electronic configurations under realistic operational and environmental constraints. The overall objective of the work reported in this paper was to develop and validate such a simulation framework, which consists of (1) a time-dependent radiative transfer model to accurately predict the channel impulse characteristics for alternate system designs over a range of geometries and optical properties and (2) digital modulation and demodulation blocks which accurately simulate both laser source and receiver noise characteristics in order to generate time domain bit stream samples that can be digitally demodulated to predict the resulting bit error rate of the simulated link.

Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields. Over the last few years, the authors have conducted extensive research on CS-based active electro-optical imaging in a scattering medium, such as the underwater environment. This paper proposes a compressive line sensing underwater imaging system that is more compatible with conventional underwater survey operations. This new imaging system builds on our frame-based CS underwater laser imager concept, which is more advantageous for hover capable platforms. We contrast features of CS underwater imaging with those of traditional underwater electro-optical imaging and highlight some advantages of the CS approach. Simulation and initial underwater validation test results are also presented.

This paper will discuss and compare some recent oceanic test results from the Bahamas Optical Turbulence Exercise (BOTEX) cruise, where vertical profiling was conducted with both time-resolved laser backscatter measurements being acquired via a subsurface light detection and ranging (lidar) profiling instrument, and laser beam forward deflection measurements were acquired from a matrix of continuous wave (cw) laser beams (i.e. structured lighting) being imaged in the forward direction with a high speed camera over a one-way path, with both transmitter and camera firmly fixed on a rigid frame. From the latter, it was observed that when within a natural turbulent layer, the laser beams were being deflected from their still water location at the image plane, which was 8.8 meters distance from the laser dot matrix transmitter. As well as suggesting that the turbulent structures being encountered were predominately larger than the beam diameter, the magnitude of the deflection has been confirmed to correlate with the temperature dissipation rate. The profiling lidar measurements which were conducted in similar conditions, also used a narrow collimated laser beam in order to resolve small-scale spatial structure, but with the added attribute that sub-nanosecond short pulse temporal profile could potentially resolve small-scale vertical structure. In the clear waters of the Tongue of the Ocean in the Bahamas, it was hypothesized that the backscatter anomalies due to the effect of refractive index discontinuities (i.e. mixed layer turbulence) would be observable. The processed lidar data presented herein indicates that higher backscatter levels were observed in the regions of the water column which corresponded to higher turbulent mixing which occurs at the first and second themoclines. At the same test stations that the laser beam matrix and lidar measurements were conducted, turbulence measurements were made with two non-optical instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both these setups in order to characterize the physical environments and corroborate with the laser measurements. To further investigate the utility of elastic lidar in detecting small-scale turbulent structures, controlled laboratory experiments were also conducted, with the objective of concurrently acquiring both the laser beam spatial characteristics in the forward direction and the laser backscatter temporal profile from each transmitted sub-nanosecond pulse. An artificial refractive index discontinuity was generated in clear test tank conditions by placing a clean ice-filled carboy above the laser beam propagation path. The results from both field and laboratory experiments confirm our hypothesis that turbulent layers are detectable by lidar sensors, and motivates that more research and lidar instrumentation development is needed to better quantify turbulence, especially for mitigating associated performance degrading effects for the U.S. Navy’s next generation electro-optic (EO) systems, including active laser imaging and laser communications.

Mobile, high throughput mid-range data communications and robust real-time data networking in the subsea environment that can accommodate high bandwidth sensors such as optical imagers have a potentially high impact as enabling technologies for a variety of future subsea operations in the areas of distributed sensing and real-time wireless feedback and control of unmanned undersea vehicles. Although much work has been done recently in the field of undersea optical free space communications and networking, to date there has yet to be an implementation of a complete multi-node undersea wireless optical data communications network. The deployment and testing of optical wireless network equipment in the undersea environment is expensive and time-consuming, and there is a clear need for a network simulation framework that will allow researchers to evaluate the performances of different networking concepts/configurations under realistic operational and environmental constraints. This paper describes a network simulation approach that uses an accurate time dependent Monte Carlo channel model to simulate the networking physical layer, which can be used in conjunction with higher network layer protocols to simulate larger scale network performance and to help determine hardware requirements for overall network system design in a variety of undersea channel conditions.

Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields.
In recent years, the FAU/HBOI OVOL has conducted extensive research to study the CS based active electro-optical
imaging system in the scattering medium such as the underwater environment. The unique features of such system in
comparison with the traditional underwater electro-optical imaging system are discussed. Building upon the knowledge
from the previous work on a frame based CS underwater laser imager concept, more advantageous for hover-capable
platforms such as the Hovering Autonomous Underwater Vehicle (HAUV), a compressive line sensing underwater
imaging (CLSUI) system that is more compatible with the conventional underwater platforms where images are formed
in whiskbroom fashion, is proposed in this paper. Simulation results are discussed.

Compressive sensing (CS) theory has drawn great interest in recent years and has led to new image-acquisition techniques in many different fields. This research investigates a CS-based active underwater laser serial imaging system, which employs a spatial light modulator (SLM) at the source. A multiscale polarity-flipping measurement matrix and a model-assisted image reconstruction concept are proposed to address limitations imposed by a scattering medium. These concepts are also applicable to CS-based imaging in atmospheric environments characterized by fog, rain, or clouds. Simulation results comparing the performance of the proposed technique with that of traditional laser line scan (LLS) sensors and other structured illumination-based imager are analyzed. Experimental results from over-the-air and underwater tests are also presented. The potential for extending the proposed frame-based imaging technique to the traditional line-by-line scanning mode is discussed.

The Bahamas Optical Turbulence Exercise (BOTEX) was conducted in the coastal waters of Florida and the Bahamas
from June 30 to July 12 2011, onboard the R/V FG Walton Smith. The primary objective of the BOTEX was to obtain
field measurements of optical turbulence structures, in order to investigate the impacts of the naturally occurring
turbulence on underwater imaging and optical beam propagation. In order to successfully image through optical
turbulence structures in the water and examine their impacts on optical transmission, a high speed camera and targets
(both active and passive) were mounted on a rigid frame to form the Image Measurement Assembly for Subsurface
Turbulence (IMAST). To investigate the impacts on active imaging systems such as the laser line scan (LLS), the
Telescoping Rigid Underwater Sensor Structure (TRUSS) was designed and implemented by Harbor Branch
Oceanographic Institute. The experiments were designed to determine the resolution limits of LLS systems as a function
of turbulence induced beam wander at the target. The impact of natural turbulence structures on lidar backscatter
waveforms was also examined, by means of a telescopic receiver and a short pulse transmitter, co-located, on a vertical
profiling frame. To include a wide range of water types in terms of optical and physical conditions, data was collected
from four different locations. . Impacts from optical turbulence were observed under both strong and weak physical
structures. Turbulence measurements were made by two instruments, the Vertical Microstructure Profiler (VMP) and a
3D acoustical Doppler velocimeter with fast conductivity and temperature probes, in close proximity in the field.
Subsequently these were mounted on the IMAST during moored deployments. The turbulence kinetic energy dissipation
rate and the temperature dissipation rates were calculated from both setups in order to characterize the physical
environments and their impacts. Beam deflection by multiple point patterns are examined, using high speed camera
recordings (300 to 1200 fps), in association with measured turbulence structures. Initial results confirmed our hypothesis
that turbulence impacted optical transmissions. They also showed that more research will be needed to better quantify
and mitigate such effects, especially for the U.S. Navy's next generation EO systems, including active imaging, lidar and
optical communications.

Recent progress in system hardware such as laser, photon detectors and other electronic and optical components resulted
in significant improvement for the underwater serial laser imaging system. Nevertheless, during normal system
operation, system issues such as laser instability, electronic noise, and environmental conditions such as imaging in
highly turbid water can still put constraint on the performance of imager. In this work, post-processing to take advantage
of the improvement hardware to further reduce image noise and enhance the image quality as a critical aspect of the
overall system design is studied. A novel realization of the bilateral principle based image/pulse noise reduction and
image deconvolution using point spread function (PSF) predicted with EODES radiative transfer model is used to
implement the processing chain. The concept is further extended to a multichannel deconvolution to exploit the benefit
offered by the new multi element PMT configuration developed in HBOI. Two datasets were used to test the developed
techniques respectively.

The compressive sensing (CS) theory has drawn great interest in signal processing community in recent years and led
to new image acquisition techniques in many different fields. This research attempts to develop a CS based underwater
laser serial imaging system. A Digital Mirror Device (DMD) based system configuration is proposed. The constraints
due to scattering medium are studied. A multi-scale measurement matrix design, the "model-assisted" image
reconstruction concept and a volume backscattering reduction technique are proposed to mitigate such constraints. These
concepts are also applicable to CS based imager in other scattering environment such as fog, rain or clouds. Simulation
results using a modified imaging model developed by HBOI and Metron and experimental results using a simple optical
bench setup are presented. Finally the proposed technique is compared with traditional laser line scan (LLS) design and
other structured illumination based imager.

This paper examines imaging performance bounds for undersea electro-optic identification (EOID) sensors that use
pulsed laser line scanners to form serial images, typically utilizing one laser pulse for each formed image element. The
experimental results presented include the use of two distinct imaging geometries; firstly where the laser source and
single element optical detector are nearly co-aligned (near monostatic) and secondly where the laser source is deployed
on a separate platform positioned closer to the target (bistatic) to minimize source-to-target beam spread, volumetric
scatter and attenuation, with the detector being positioned much further from the target. The former system uses
synchronous scanning in order to significantly limit the required instantaneous angular acceptance function of the
detector and has the desired intention of acquiring only ballistic photons that have directly interacted with the target
element and the undesirable property of acquiring snake photon contributions that indirectly arrive into the detector
aperture via multiple forward scattering over the two-way propagation path. The latter system utilizes a staring detector
with a much wider angular acceptance function, the objective being to deliver maximum photon density to each target
element and acquire diffuse, snake and ballistic photon contributions in order to maximize the signal.
The objective of this work was to experimentally investigate pulse-to-pulse detection statistics for both imaging
geometries in carefully controlled particle suspensions, with and without artificially generated random uncharacterized
scattering inhomogeneities to assess potential image performance in realistic conditions where large biological and
mineral particles, aggregates, thin biological scattering layers and turbulence will exist. More specifically, the study
investigates received pulse energy variance in clear filtered water, as well as various well-characterized particle
suspensions with and without an artificial thin random scattering layer. Efforts were made to keep device noise constant
in order to assess the impact of the environment on extrapolated image quality.

The objective of this work was to develop and validate approaches to accurately and efficiently model channel
characteristics in a range of environmental and operational conditions for underwater laser communications systems that
use high frequency amplitude modulation (AM) or coded pulse trains. Two approaches were investigated: 1) a Monte
Carlo model to calculate impulse responses for a particular system hardware design over a large range of environmental
and operational conditions, and 2) a semi-analytic model which has the potential to be more computationally efficient
than the Monte Carlo model. The formulation of the Monte Carlo code is presented in this paper, together with test
results used to evaluate the range of accuracy of the model against 500ps laser-pulse propagation measurements from
well-controlled and characterized particle suspensions in a 12.5m test tank. A multiple scattering study using the Monte
Carlo simulation code was also performed and some results are presented. Results from the semi-analytic model will be
compared with these test tank measurements and the Monte Carlo model in a later paper.

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